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Structural Engineering Rules of Thumb
5

Structural Engineering Rules of Thumb

Structural Engineering Rules of Thumb

(OP)
There have been some great posts skirting around this subject, however I really think we need a Tips & Tricks FAQ.  With that goal in mind, I thought it would be good if we all posted the routine rules of thumb we use in our daily work, in order to perhaps learn some useful shorthand and tips or tricks.

Structural Engineering Rules of Thumb:

1.  Beams need to have shear effects considered specifically (increased deflection, possibly reinforcement requirements in concrete, etc) when SPAN/DEPTH <=10

2.  Beam design is normally deflection governed when SPAN/DEPTH >=25.

3.  When checking drawings, looking at Moment/Plastic Section Modulus is a good guage of a section's sizing, even if using Limit States Design.  Knowing the extreme fibre stress is a good "feel" for the beam size.

4.  Always consider a minimum accidental eccentricity of 100mm in your construction.  Increase this to 150mm in residential work.

5.  Design shelf angles for the load at the very tip for strength (ULS), centre of bearing for serviceability (SLS).  This ensures that any rotation of the beam at the support does not lead to overstress in the fixing; Particularly for stiffened angles.

6.  When in doubt, add confinement to concrete.  Curtailment of reinforcing should occur at a distance of 130% development length past the point where strength is last required, or Ld+d from support, whichever is greater.

7.  To minimize the risk of timber floors (and all high frequency floors; Applies to Cold Formed Steel as well), check that the deflection is no greater than 1 to 2mm under a 1kN point load at centre.  Do not consider T-Beam stiffening effect for this check unless the plywood is glued and screwed; slip and fastener loosening may not permit adequate composite action otherwise.

8.  For steel and concrete beams, check the estimated natural frequency, equal to 18/SQRT(Total Deflection in mm), result in hertz (HZ).  Use anticipated actual loads in this check (thus typically 0.25kPa to 0.35 kPa) rather than full SLS loads.  A result of 15Hz or higher should be double checked with the point load check, a result between 8HZ and 15 HZ is likely okay, with likihood of difficulty increasing as the result decreases, and anything between 5HZ and 8HZ should be subject to a full accelerative methodology vibration check (such as the ATC guideline or CISC Guide 11).  Picking the loading is very important, and entirely subjective; A good guide is to consider 30% of your floor load as the likely "routine" load.   That way you are basing the load used on the code's anticipated exposure loads for the floor type.  Remember that vibration problems normally happen under light loading.

9.  For good ground checks in the field, get a metal or timber block made up which should impose the Ultimate Bearing Pressure required of the soil.  Stand on this for a count of five anywhere you have some doubt over good ground.  An indentation of anything more than a mark (so say greater than 1mm) is considered a failure.  Example:  I weight 100kg, my typical "Good Ground" value is 300 kPa Ultimate (rupture) bearing pressure, thus my block is roughly 57mm by 57mm.

I'll keep posting as I think of more.
Cheers all,

YS

 

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

RE: Structural Engineering Rules of Thumb

Good start.
Points 1 and 2 are material specific?
3 - Agreed.  In UK we have a limit state code (similar to LRFD) The section properties are given in a separate document (Blue Book) which can readily be used for simple cross checks.  Checking designs could really do with a whole topic of its own...
Point 9 - I like this.  I've never done it.  It's only likely that I'd be responsible for this sort of thing on small projects but I might just make up some blocks to test it out.  Guess I'll need to weigh myself.  Maybe later...

RE: Structural Engineering Rules of Thumb

A good rule of thumb our office uses is stay away from the metric system...hahah.

RE: Structural Engineering Rules of Thumb

If any of you have been or are planning on going to the NASCC, you may have heard of the course "Rules of Thumb for Steel Design" given by Socrates Ioannides of Structural Affiliates International.  Well worth the hour and a half. Plus he has a handy pocket quide that he hands out at the end.  

 

RE: Structural Engineering Rules of Thumb

Concrete slabs:  20 to 25 to 1 span to depth ratio
Concrete beams:  2 to 1 depth to width ratio

 

Mike McCann
MMC Engineering

RE: Structural Engineering Rules of Thumb

As = M/4d

inches of course

RE: Structural Engineering Rules of Thumb

Does anyone have a rule of thumb for the aspect ratio of a diaphragm in which the chord forces can be neglected??

RE: Structural Engineering Rules of Thumb

What's an inch? Is it one of those archaric units no longer used in the advanced world?

RE: Structural Engineering Rules of Thumb

25.4mm per inch.

I'm going to upset people but the US and other nations still using imperial units should convert to metric.

It makes much more sense, it is all based on water.

RE: Structural Engineering Rules of Thumb

youngstructural

Excellent post and I would support an FAQ section on rules of thumb.  Here's a contribution.

Base width of a stable retaining wall is typically 55% of retained height. (considering zero passive pressure at the front of wall, a level backfill and zero surcharge.)

RE: Structural Engineering Rules of Thumb

A column with as load at minimum (100mm) eccentricity can take half the load of a concentrically loaded column of the same size.

RE: Structural Engineering Rules of Thumb

RWF7437
Will you accept; it's all based on the decimal system?

RE: Structural Engineering Rules of Thumb

I have to give apsix a bit of support here.  While it may not strictly be based on water, there are a lot of useful relationships with water, particularly for structural engineers.

A kilogram of water occupies a liter.
A tonne of water occupies a cubic metre.
A metre deep water imposes 10 kPa pressure. (better than remembering that a foot of water is 62.4 psf)

RE: Structural Engineering Rules of Thumb

Maybe we should all go back to the imperial system.

RE: Structural Engineering Rules of Thumb

With metric you dont have to keep dividing by 12 and 8ths e.t.c. all the time!

Been there, done that. Back to the country that invented the inch but which has since then moved on.

RE: Structural Engineering Rules of Thumb

The metric vs. imperial debate is similar to the ASD vs. LRFD debate. Yes- there is probably an argument to be made for each side, but you dance with the one that brung ya'.

I grew up using both sets of units. As soon as you spend an extended amount of time in one system, you kind of loose a feel for what is reasonable in the other set.

At the end of the day, we are engineers and we should be able to work with either set of units. I currently have a good feel for what works in imperial units, but were I to change locations and begin using metric, I'm sure it would just be a matter of time before I developed that "feel".

And please stop the "you don't have to keep dividing by 12.." b.s.  That's about as irrelevant as the argument that you have to check "a whole other set of load combinations to check deflection if you use LRFD".

RE: Structural Engineering Rules of Thumb

Why is "you don't have to keep dividing by 12 and 8" irrelevant?  It's an additional step (or two) that is a potential source of error when doing hand calcs.

"Feel" will be the same for either system.  But there are clear advantages, seen by most of the world, for using a decimal measurement system to go with our decimal numbering system.  I've lived in both systems too (daily life as well as calculations on paper), and the advantage of SI is abundantly clear.

On the other hand, the U.S. system is what it is, and this forum is not going to convince the U.S. industrial base to switch.  Economics will drive it, and the change is happening already, albeit at a glacial pace.  There's no point in lecturing engineers who have no choice as to which system to work in that they ought to be using a different system.

Hg

Eng-Tips policies:  FAQ731-376: Eng-Tips.com Forum Policies

RE: Structural Engineering Rules of Thumb

It's irrelevant because if you make an error converting from feet to inches, you shouldn't be practicing engineering.

If you (the "royal" you, not you personally) can't handle a single conversion, I suspect your work is littered with incorrect assumptions, bad judgement and mathematical errors anyway.

RE: Structural Engineering Rules of Thumb

I don't think being able to "handle" a feet to inch converstion is the point.  Additional steps in any dimensional analysis are sources for potential calculation error.  I think every engineer having worked with Imperial units has made this type of common blunder at one time or another.  

Of course, there are always those engineers that don't have erasers on their pencils and feel compelled to convince everyone else to remove the delete and backspace keys from their keyboards since they are so obviously superfluous.

RE: Structural Engineering Rules of Thumb

Respectfully, one of you should start a separate "which units are better?" thread instead of taking this one off on a tangent.

Rule of thumb:  Ft-weight of steel beam= 3.6M/d (M in kip-ft, d in inches)  

JK

 

RE: Structural Engineering Rules of Thumb

JKStuct

You are correct indeed. My apologies.

Rule of Thumb:
Control Joint spacing for Concrete Slabs on grade:  
Non-reinforced 2*t(slab thickness)= CJ spacing in ft,
Reinforced 7*t @ t=4", 4*t @ t=5", 3*t @ t=6", 6x6-W1.4xW1.4.
 

RE: Structural Engineering Rules of Thumb

Back to the OP:

RE: point 9 - good ground checks

When checking the preparation of the sub-grade for a slab, road, etc, take a golf ball along, and bounce it as you wander around the site. (You don't have to throw it down hard - just casually bounce it in rhythm with your pace, just as you might while walking along a footpath.) Whenever it fails to bounce back up to your hand, mark the spot for compaction testing.

RE: Structural Engineering Rules of Thumb

drag a plastic (or steel) chains across decks to sound for cracks.

RE: Structural Engineering Rules of Thumb

1 inch per foot of unbraced length for concrete columns.

RE: Structural Engineering Rules of Thumb

YS,

It is a good list I will pass it around the office. Although Item 9 is interesting we basically design for soil types and their relative bearing capacity and later confirm during excavation. For larger projects we insist on Geotech report.

 

Neilly Davies Consulting Engineers http://www.neillydavies.com.au

RE: Structural Engineering Rules of Thumb

Very interesting post.
I don't have much to contribute that beyond all have been said, only to add a little reminder:

"Do not blindly apply any RULE-OF-THUMB until one has accumulate enough knowledge in his/her field of work, and fully understand how and where the RULEs came about".

Sorry to be a spoiler of a wonderful party.

RE: Structural Engineering Rules of Thumb

Rules of thumb make good starting points.  Then run the numbers.  They also make good sanity checks after the numbers have been run.

Hg

Eng-Tips policies:  FAQ731-376: Eng-Tips.com Forum Policies

RE: Structural Engineering Rules of Thumb

(OP)
kslee:

You haven't harmed the party at all... I think we often assume everyone's going to be rational and run the numbers, but that's just not always the case.  It's good to have someone actually say it.

One more from my experience:

- Retaining walls should be attempted with "traditional" dimensions first, and make every effort to correctly size and balance the heel and toe.  There are good reasons why these shapes (toe to heel from 0.45 of height to 0.55, etc) are so commonly found.  Stability, sliding, etc are easy to satisfy with an oversized heel or toe, but the strength of these members will be very difficult to achieve.

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

RE: Structural Engineering Rules of Thumb

Thickness of side walls for on-ground closed water retaining structure=height/10.
 

RE: Structural Engineering Rules of Thumb

sfat,

That is a very misleading rule of thumb.  Depends a lot on the type of structure, and in most cases, I would say it is overly conservative.

RE: Structural Engineering Rules of Thumb

Deflection of a beam, d=Mf.L^2/(10EI), where
.  Mf= Midspan moment (Nmm)
.  L=Span (mm)
.  E=Young's modulus (MPa)
.  I=Moment of Inertia (mm^4)
N,mm or any compatible set of units.

RE: Structural Engineering Rules of Thumb

Most of my serious design stuff ups have begun with the senior engineer saying "This is straight forward, no need to think about it, just get to work."

RE: Structural Engineering Rules of Thumb

@SDZ,
Bingo, it usually starts like that. I must tell you that good mentoring is hard to find, you just have to be really lucky. Most of the time, you have to figure it out yourself.

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